The adsorption of two azo dyes, namely naphthol red-J and direct orange, on nontronite mineral was studied as a function of pH and temperature. All the sorption studies were conducted in batch mode. The Langmuir and Freundlich isotherm models were applied. The isotherms are Langmuirian in nature, while the Freundlich equation is only valid over the low concentration range. The adsorption of the two dyes increases with increased temperature and decreases with increased pH. The data have been explained in terms of the area of contact of the dye molecule on the clay platelet vis-a-vis the area per anion exchange site. Estimated area per anion exchange site of the mineral and area associated per anionic dye molecule at nontronite surface was 642 and 454 angstrom(2) for naphthol red-J and 642 and 440 angstrom(2) for direct orange respectively. This has been further supported by X-ray measurements. Thermodynamic parameters of the process were evaluated. (c) 2005 Elsevier Inc. All rights reserved.